The present invention relates to adhesion promoters which are used to incorporate pigments into thermoplastics.
It is known that, when pigmenting granulated thermoplastics which are processed, for example, in injection molding machines or extruders to give pigmented shaped articles, adhesion promoters are added to the mixture. These additives are intended in particular to effect a uniform distribution of the pigments in the plastic melt.
Particularly in the case of pearlescent pigments based on mica flakes coated with metal oxides, in particular titanium dioxide, which are frequently employed for pigmenting thermoplastics, difficulties often arise because the hydrophilic pigment aggregates are difficult to disperse into the highly viscous plastic melts, which in most cases are non-polar. There results a non-uniform distribution of the pigments in the plastics which have been extruded, injection molded or processed in other ways.
The addition of low-molecular weight polyethylene wax is recommended in DE-A No. 2,258,729 in order to improve the incorporation process. Although this additive effects a reduction in the viscosity of the medium primarily enveloping the pigment particles, at the same time the polarity of the latter becomes even lower, so that the remaining effect is only a slight improvement.
It has also been suggested to add a solvent which acts as a plasticizer for the plastic to the mixture of pigment/plastic granules. However, in this procedure too, numerous pigment agglomerates can be seen after processing, even with the naked eye.
Because the pigmentation is unsatisfactory in these respects, the pigments have, therefore, frequently been employed in the form of a master batch or a pre-granulation has been carried out before the actual processing to give shaped articles. Pigment/plastic mixtures are processed via an extruding/granulating machine to give pigmented granules and these granules are then shaped, for example by the injection molding process.
However, both the preparation of a master batch and pre-granulation constitute a considerable further outlay of time and equipment capacity, and this considerably increases the cost of production.
There is, therefore, a need for an improved process.